Catalytic domain of the Protein Tyrosine Kinase, C-terminal Src kinase. Protein Tyrosine Kinase (PTK) family; C-terminal Src kinase (Csk); catalytic (c) domain. The PTKc family is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, and phosphoinositide 3-kinase (PI3K). PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. The Csk subfamily kinases are cytoplasmic (or nonreceptor) tyr kinases containing the Src homology domains, SH3 and SH2, N-terminal to the catalytic tyr kinase domain. They negatively regulate the activity of Src kinases that are anchored to the plasma membrane. To inhibit Src kinases, Csk is translocated to the membrane via binding to specific transmembrane proteins, G-proteins, or adaptor proteins near the membrane. Csk catalyzes the tyr phosphorylation of the regulatory C-terminal tail of Src kinases, resulting in their inactivation. Csk is expressed in a wide variety of tissues. As a negative regulator of Src, Csk plays a role in cell proliferation, survival, and differentiation, and consequently, in cancer development and progression. In addition, Csk also shows Src-independent functions. It is a critical component in G-protein signaling, and plays a role in cytoskeletal reorganization and cell migration.